12 ALDEHYDES & KETONES class xii chemist

class xii chemistry aldehydes, ketones and carboxylic acids ppt

1. ALDEHYDE KETONES AND CARBOXYLIC ACIDS

2. Session Objectives Introduction Nomenclature of aldehydes Physical properties Preparation of aldehydes Chemical reactions Nucleophilic reactions Reduction oxidation Reactions involving a- hydrogen

3. Carbonyl Structure Carbon is sp 2 hybridized. C=O bond is shorter, stronger, and more polar than C=C bond in alkenes.

4. General methods of preparation From alcohols by oxidation by dehydrogenation From acid chlorides From nitriles From hydrocarbons by ozonolysis of alkenes by hydration of alkenes by oxidation of methylbenzenes

5. By dehydrogenation of alcohols This method is suitable for volatile alcohols and is of industrial application .

6. From acyl chloride (acid chloride) Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate . This reaction is called Rosenmund reduction.

7. From nitriles and esters This reaction is called Stephen reaction.

8. Alternatively, nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehyde .

9. From hydrocarbons Aromatic aldehydes ( benzaldehyde and its derivatives) are prepared from aromatic hydrocarbons by the following methods: ( i ) By oxidation of methylbenzene Strong oxidising agents oxidise toluene and its derivatives to acids:

10. (a) Use of chromyl chloride (CrO2Cl2): Chromyl chloride oxidises methyl group to a chromium complex, which on gives corresponding benzaldehyde. This reactionhydrolysis is called Etard reaction.

11. (b) Use of chromic oxide (CrO3): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid.

12. (ii) By side chain chlorination followed by hydrolysis Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde . This is a commercial method of manufacture of benzaldehyde .

13. ( iii) By Gatterman – Koch reaction When benzene or its derivative is treated with carbon monoxideand hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde .

14. PHYSICAL PROPERTIES Physical State - Methanal is a gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature. Boiling Point –The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions . Also, their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding

15. Solubility The lower members of aldehydes and ketones such as methanal , ethanal and propanone are miscible with water in all proportions, because they form hydrogen bond with water.

16. CHEMICAL PROPERTIES Since aldehydes and ketones both possess the carbonyl functional group, they give following reaction: Nucleophilic Addition Reaction Nucleophilic Addition Reaction followed by removal of water molecule Oxidation Reduction Miscellaneus

17. Nucleophilic Addition A strong nucleophile attacks the carbonyl carbon, forming an alkoxide ion that is then protonated. A weak nucleophile will attack a carbonyl if it has been protonated, thus increasing its reactivity. Aldehydes are more reactive than ketones.

18. The reaction of Grignard reagents with methanal produces a primary alcohol, with other aldehydes , secondary alcohols and with ketone tertiary alcohols

19. Addition of Alcohol In presence of dry HCl aldehydes and ketones react with two equivalent of alcohols to form acetals and ketals

20. Addition of ammonia and its derivatives Nucleophilic addition of ammonia or primary amine, followed by elimination of water molecule. C=O becomes C=N-R

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22. Oxidation Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens ’ reagent and Fehlings ’ reagent also oxidise aldehydes .

23. Ketones Oxidize with Difficulty Undergo slow cleavage with hot, alkaline KMnO 4 C–C bond next to C=O is broken to give carboxylic acids. Cleavage at C-C bond gives mixture of acids.

24. Tollens Test Add ammonia solution to AgNO 3 solution until precipitate dissolves. Ammonical silver nitrate solution [Ag(NH3)2]OH is called Tollens reagent. Aldehyde react with Tollens reagent forms a silver mirror.

25. Silver Mirror

26. Fehling’s test

27. iii.haloform ( iodoform )

28. Reduction Reagents Sodium borohydride , NaBH 4 , reduces C=O, but not C=C. Lithium aluminum hydride, LiAlH 4 , much stronger, difficult to handle. Hydrogen gas with catalyst also reduces the C=C bond.

29. Clemmensen Reduction

30. Alpha hydrogen-acidity

31. Cross – aldol condensation

32. 3.Electrophilic substitution